How would I make perfect diagonal edges in a rectangle
One way to get diagonal based lines like that is to use Offset Edge Slide which will add loopcuts on both sides of the present edge. However one edge is needed, so select opposite vertices and press J to join them with the edge and split the face.

Then select the edge, open Toolshelf (or press Ctrl+Shift+R) and drag the mouse for the effect.



Regarding the edge you create for this tool to work.
You can dissolve it afterwards but note that it creates quite horrible Ngon in the middle which is better to avoid. It all depends on the purpose of the model but it's best to stay away from this geometry to avoid any problems later on.